Apparatus for testing lenses



F. TWYMAN.

APPARATUS FOR TESTING LENSES.

APPLICATION man APR. 23, 1919.

Patented July 20, 1920.

4 SHEETS-SHEET 1.

F. TWYMAN. APPARATUS FOR TESTING LENSES.

APPLICATION FILED APR. 23. I919- F. TWYMAN.

APPARATUS FOR TESTING LENSES.

APPLICATION FILED APR. 23, 1919.

1,347, 1 33 Patented July 20, 1920.

4 SHEETS-SHEET 3.

F. TWYMAN.

APPARATUS FOR TESTING LENSES.

APPLICATION FILED APR. 23- I9l9.

1,347, 1 33 Patented July 20, 1920.

4 SHEETS-SHEET 4- UNITED STATES PATENT OFFICE.

FRANK T'WYMAN, OF LONDON, ENGLAND, ASSIGNOR T0 ADAM HILGEB, LIMITED, 0}?

' LONDON, ENGLAND.

Specification of Letters Patent.

Patented July 20, 1920.

original application filed February 4, '1919, Serial No. 274,843.Divided and this application filed April 23, 1819. Serial No. 292,042.

To all whom it may concern:

Be it known that I, FRANK TWYMAN, a subject of the King of GreatBritain, residing at 75 Camden road, London, England, have invented anew and useful Improved Apparatus for Testing Lenses, of which thefollowing is a specification.

The application for this patent is a division of my application forUnited States patent, Serial No. 274,843, filed 4th Februar 1919.

his invention relates to improvements in the apparatus for use in theprocessof testing lenses, described in the specification of the formerapplication, Serial No. 274,843.

According to this invention, the lens is tested and corrected not onlyfor beams of light entering the lens in an axial direction, but also forbeams entering in oblique directions. The lens may be first tested andcorrected for axial beams and subsequentlytested and correctedfor'oblique beams. Or, since the correction for axial beams has effecton the oblique beams and vice versa, it is in practice simpler to makepartial corrections for axial and oblique beams alternately until thegreatest possible perfection is attained.

In an apparatus for carrying out this invention the lens is so mountedupon a carriage that its position can be adjusted both along and atright angles to its optical axis and the carriage can be turned about anaxis at right angles to the optical axis of the lens. By adjustment ofthe lens the second nodal point of the lens can be positioned preciselyin the axis about which the carriage turns.

The rotation of the carriage is effected by means of a rod which extendsto the outer edge of the interferometer and is parallel to the axis ofthe lens, and a scale is provided so that the angle through which thelens is turned can be measured. Upon the rod and at right angles to itis mounted a bar whose distance from the axis of rotation of the lenscan be adjusted, a scale being provided on the rod to enable thedistance of the bar from the axis of rotation to be noted. The mirror(which may be either concave or convex) by which the light is reflectedback through the lens is adjustably mounted on a carriage, and thecarriage is automatically moved toward or away from the lens so that thecenter of curvature of the mirror is always in the focal plane in whichthe lens is supposed to form its image. In order to accomplish this aflexible connection is led from the carriage to a weight, and upon thecarriage is a lug which is retained continually in contact with the barabove mentioned by the action of the weight.

The annexed drawings show an apparatus made in accordance with thisinvention. F 1gure 1 is a plan, Fig. 2 a side elevation partly Insection of the apparatus, and Figs. 3 and 4 are a plan and sideelevation to a larger scale of the carriageupon which the lens ismounted. Figs. 5 and 6 are similar views to Figs. 3 and 4 of a carriageupon which the mirrors are mounted. Fig. 7 is a diagrammatic viewshowing a lens under tes Light from a suitable source is reflectedtoward the apparatus by means of a mirror 10 and is condensed by a lens11 on to the aperture of a diaphragm 12 through which it passes afterbeing rendered parallel by a lens 121 to a plane parallel glass plate13. The surface of the glass plate is partially silvered so that part ofthe lightis reflected and part transmitted by it. The transmittedportion of the light passes to a lens 14 which is under test and 15 is amirror by which the light is reflected back through the lens 14 to theplate 13 by which it is partly reflected toward the diaphragm 16 andforms an image of the aperture in the diaphragm 12. The other part ofthe beam passes to the mirror 17 from whence it is reflected back to theplate 13, part of the beam passing through the plate and combining withthe beam that has passed through the lens 14.

The lens 14 is carried by a frame 141 free to move in a line at rightangles to the optical axis of the lens upon a block 142 by means of ascrew 140. The block 142 is mounted on a carriage 143 upon which it canbe moved along the optical axis of the lens by means of a screw 144. Themirror 15 is mounted on a frame 151 capable of movement upon a plate 152the frame 151 being clamped to the plate 152 by screws 153, see Figs. 5and 6. The plate 152 is mounted on a carriage 154 and is capable ofmovement upon it and is held in position by a screw 155.

' obliquely The carriage 143 is turned upon a pivot 191 by a rod 19 edgeof the apparatus over a scale 192 which indicates the number of degreesthrough which the rod is turned.- When the carriage 143 is rotated onthe pivot, the lens 14 is correspondingly rotated so that if originallythe incident beam is parallel to the axis of the lens in its firstposition (such as the position shown in Fig. 1) the same beam will passthrough the lens obliquely relatively to the lens when the lens isrotated. If the lens 14 were perfect all of the rays passing through thelens would travel in a direction parallel to the direction of travelbefore their incidence on the lens. In other words, the rays passingthrough a perfect lens obliquely thereto are not materially deflectedfrom the direction of their travel except that the rays are brought to afocus. However, the focal plane of the lens moves away from the lens adistance proportionate to the angle through which it has been turned.Hence, since it is necessary in an apparatus of this kind that the focalplane of the lens corresponds to the center of curvature of the mirrorit is necessary to move the mirror away from the lens a distanceproportionate to the distance the focal plane of the lens is moved awayfrom the lens, this distance being proportionate also to the angle ofrotation of the lens. For effecting this movement of the mirrorautomatically, I use the apparatus hereinafter described.

At right angles to the rod 19 is adjustably secured a bar 20 and thecarriage 154 is capableof sliding above the rod 19 and has attached toit by a flexible connection 21, a weight (not shown) which weight alwaystends to maintain a. lug 155 upon the car-- riage 154 in contact withthe bar 20. The mirror is in this way moved toward and away from thelens'a distance corresponding to the angle of rotation of the latter, sothat the center of curvature of the mirror is always in the plane inwhich it is desired that the lens should focus its images; to maintainthe mirror in this focal plane it must be moved a distance equal to thedistance marked 23 as is shown in Fig. 7. If

the center of curvature of the mirror were not in the focal plane of thelens, 11. e., in the plane in which it is desired that the foci of thevarious lenses should lie-then the reflected image would not fall in thesame plane as the focal plane of the lenses and consequently even if aperfect lens were used.

- a distortion would be apparent at the eye piece 16. By stating that itis desirable to so adjust the apparatus that the center of curvaturecorresponds with the focal plane of the lens, itis contemplated that theadjustment be originally made from a perfect lens and that other lensesbe corrected from which extends to the outerwhich the carriage turns.

the adjustment thus first obtained. In full means of cords 173. 174 is alens through which the recombined beams from the mirrors 15 and 17 pass.

On the plate152 is also mounted a frame 181 in which is another mirror18, the frame 181 being clamped to the plate 152, by screws 182. Themirror 15 is convex and the mirror 18 is concave, the mirror 15 beingremoved wheni lenses of short focal length are being teste When it isdesired to test a lens it is placed in the frame 141 on the carriage 143and is so adjusted that the second principal point of the lens ispositioned in the axis about The mirror is then suitably adjusted andthe interference rings upon the lens are observed and the parts of thelens requiring polishing are either marked on the lens or on a map ofthe same. When the parts of the lens have been polished with a view totheir partial removal the lens is again mounted upon the carriage andthe rod 19 is turned so that the lens may be examined for obliquepencils, the interference rings are observed and the lens is partiallytreated to remove the same a and is then alternately treated for axialand oblique pencils, until the desired accuracy is attained.

What I claim is:-

1. An apparatus for testing a 'lenseom-- prising a plane parallel glassplate partially silvered and adapted to reflect part of a beam of lightand to transmit a part, two mirrors adapted to reflect the light back tothe plate and mounted so that their distances from the plate can bevaried, a car riage adapted to adjustably receive the lens tobe testedand maintain the same in the path of light to and from one of saidmirrors, means for moving the lens on the carriage both along and atright angles to its optical axis and means for turning the carr1a aboutan axis at right angles to the optical axis of the lens.

2. An apparatus for testing lenses comprising a plane parallel glassplate partially and from one of said mirrors, means for turning thecarriage about an axis at right angles to the optical axis of the lens,and means for adjusting the mirror by which the light is reflected backthrough the lens, so that the center of curvature thereof is in thefocal Elane of the lens.

3. n apparatus for testing lenses comprising a plane parallel glassplate partially silvered and adapted to reflect part of a beam of lightand to transmit a part, two mirrors adaptedto reflect the light back tothe plate and mounted so that their distances from the plate can bevaried, a carriage adapted to receive the lens to be tested and maintainthe same in the path of light to and from one of said mirrors, means formoving the lens on the carriage both along and at right angles to itsoptical axis, means for turning the carriage about an axis at rightangles to the optical axis of the lens, and means for adjusting themirror by which the light is reflected back through the lens, so thatthe center of curvature thereof is in the focal plane of the lens.

4. An apparatus for testing lenses comprising a plane parallel glassplate partially silvered and adapted to reflect part of a beam of lightand to transmit a part, two mirrors adapted to reflect the light back tothe plate and mounted so that their disfor maintaining the carriage ofsaid mirror by which the light is reflected back through the lens incontact with the bar.

5. An apparatus for testing lenses, comprising a member adapted toreflect part of a beam of light and transmit a part, two mirrors adaptedto reflect the light back to said member, means for moving the mirrorstoward and from said member, an adjustable carriage adapted to receivethe lens to be tested and maintain the same in the path of light to andfrom one of said mirrors, means for moving said carriage in differentdirections with relation to the optical axis of the lens, and means forturning the carriage about an axis intersecting the optical axis ofthelens.

In testimony that I claim the foregoing as my invention I have signed myname this first day of April, 1919.

FRANK TWYMAN.

